Two-part back cap for a plug valve and plug valves incorporating same

ABSTRACT

A two-part back cap for a plug valve includes a back cap seal body and a back cap retainer nut. The back cap retainer nut secures the back cap seal body within the plug valve and securely supports the back cap seal body. A gap between an outer periphery of the back cap retainer nut and the back cap seal body permits the back cap seal body to flex and expand with a plug valve body of the plug valve. This ensures that the back cap seals are not stressed and a longer service life results.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/347,445 filed Dec. 31, 2008, which is a continuation of U.S. patentapplication Ser. No. 11/430,605 filed May 9, 2006, now U.S. Pat. No.7,481,418 which issued on Jan. 27, 2009.

FIELD OF THE INVENTION

This invention generally relates to fluid flow control valves and, inparticular, to back caps for high-pressure plug valves.

BACKGROUND OF THE INVENTION

High-pressure plug valves are well known in the art and used principallyin the oil and gas industry. A high-pressure plug valve is described inAssignee's co-pending United States Patent Application published underpublication number 2006/0027779 on Feb. 9, 2006, which is entitledHigh-Pressure Plug Valve with Replaceable Inserts and Method ofRefurbishing Same.

FIG. 1 shows a side elevational view of a plug valve 10 of this type.The plug valve 10 includes a plug valve body 12 having a top flange 14and a stud pad 16. As will be understood by those skilled in the art,the flange 14 and stud pad 16 could be any of a flange, stud pad orthreaded union. The plug valve 10 has a stem 18 for rotating a plug (notshown) within the plug valve body 12 to control fluid flow through thevalve in a manner well known in the art. The plug is retained in theplug valve body 12 by a back cap 20.

FIG. 2 is a schematic cross-sectional view of a back cap in accordancewith the prior art. The back cap 20 has an inner end 22 and an outer end24. The outer end 24 includes a plurality of radial bores 26 throughwhich a tool can be inserted to install the back cap 20 in the plugvalve body 12 or remove it from the plug valve body 12. The inner end 22includes a plug support surface 28 which supports the plug of the plugvalve 10. The inner end 22 further includes a seal rim 30 having aperipheral O-ring groove 32 that receives an O-ring for providing a sealabove pin threads 35 that engage box threads (not shown) in the plugvalve body 12 to retain the back cap 20 in the plug valve body 12. Theinner end 22 further includes a seal bore 36 which receives a fluid sealfor sealing around a stud end of the plug. The stud end of the plug isreceived in a guide bore 38 that aligns the plug within a plug cage (notshown) of the plug valve body 12. The stud end also extends through astud bore 40 for fluid pressure balance with the stem 18 shown in FIG.1.

A problem with the back cap 20 occurs during installation of the backcap 20 in the plug valve body 12. Because the back cap 20 is retained inthe plug valve body 12 by the pin threads 35, it must be rotated duringinstallation. When the O-ring 34 encounters a sealing surface againstwhich it seals in an interior of the plug valve body 12, the O-ring 34resists rotation due to frictional engagement with the sealing surface.Consequently, the O-ring 34 can be damaged, or it may roll partially outof the O-ring groove 32. If this occurs the valve will leak around theback cap, even though the leak may not develop until the valve has seena certain amount of high-pressure service.

Even if the O-ring 34 stays in the O-ring groove 32 and is not damagedduring installation, a leak around the back cap can still develop due tostress on the O-ring 34. As understood by those skilled in the art, whensubjected to high fluid pressures the plug valve body 12 expands, or“grows” as it is referred to in the industry. As the plug valve body 12expands, a gap develops between an outer peripheral surface of the sealrim 30 and the seal surface of the plug valve body 12. Even though anundercut 31 is provided in the back cap 20 to permit the seal rim 30 toexpand with the plug valve body 12, experimentation as shown that undervery high pressures the seal rim 30 does not expand at the same rate asthe plug valve body 12. This creates a space between the seal rim 30 andthe plug valve body 12 that puts stress on the O-ring 34. That stresseventually causes the O-ring 34 to fail, which results in a leak aroundthe back cap 20, requiring maintenance of the plug valve 10.

There is therefore a need for a back cap for a plug valve that providesa long service life without maintenance. There is also need for a plugvalve with a back cap that can be installed without danger of damagingback cap seals.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a back cap for ahigh-pressure plug valve that is easily and reliably installed andprovides a long service life without maintenance. It is a further objectof the invention to provide a plug valve with a back cap thatconsistently provides a reliable high-pressure seal.

The invention therefore provides a two-part back cap for a plug valvebody, comprising: a back cap seal body having an inner end and an outerend, the inner end comprising a plug support surface for supporting aplug in the plug valve body, and the outer end comprising first andsecond surfaces respectively inclined at respective first and secondangles towards a seal rim along an outer edge of the inner end; and aback cap retainer nut that retains the back cap seal body in the plugvalve body and has an inner surface that mates with only the firstsurface of the back cap retainer nut to permit the back cap seal body toflex under fluid pressure until the second surface contacts the innersurface of the back cap retainer nut.

The invention further provides a plug valve, comprising: a back cap sealbody having an inner end with a plug support surface for supporting aplug received in a plug valve body of the plug valve and an outer endcomprising first and second surfaces inclined at respective first andsecond angles; and a back cap retainer nut with threads that engage theplug valve body to retain the plug and the back cap seal body within theplug valve body, and an inner surface that supports only the firstsurface until the plug valve is subjected to enough fluid pressure toflex the back cap seal body and force the second surface against theinner end of the back cap retainer nut.

The invention yet further provides a two-part back cap for a plug valvebody, comprising: a back cap seal body having an inner end with a valveplug support surface, a seal rim that supports at least one O-ringagainst an end cap seal surface of the plug valve body, and an outer endhaving a first surface adjacent a stud bore through the back cap sealbody and a second surface continuous with the first surface, the firstsurface being inclined toward the seal rim at a first angle and thesecond surface being inclined toward the seal rim at a second angle thatis greater than the first angle; and a back cap retainer nut having athread for engaging the plug valve body to retain the back cap seal bodyand the plug within the plug valve body, the back cap retainer nuthaving an inner end that supports only the first surface until the backcap seal body flexes under fluid pressure and the second surface isforced into metal-to-metal contact with the inner end of the back capretainer nut.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus generally described the nature of the invention, referencewill now be made to the accompanying drawings, in which:

FIG. 1 is a side elevational view of a plug valve in accordance theprior art;

FIG. 2 is a cross sectional view of a back cap for the plug valve shownin FIG. 1;

FIG. 3 is a cross-sectional view of a two-part back cap for a plug valvein accordance with the invention;

FIG. 4 is a cross-sectional view of another embodiment of the two-partback cap in accordance with the invention;

FIG. 5 is a detailed cross-sectional view of one side of the two-partback cap shown in FIG. 3;

FIG. 6 is a detailed cross-sectional view of the side of the two-partback cap shown in FIG. 5 installed in a plug valve in accordance withthe invention; and

FIG. 7 is a detailed cross-sectional view of the two-part back cap shownin FIG. 6 when the plug valve is subjected to very high fluid pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a two-part back cap for a plug valve thatprovides a reliable long-term seal without maintenance. The two-partback cap includes a back cap seal body that can be inserted into a plugvalve body without rotation to ensure that an O-ring seal between theback cap seal body and the plug valve body is not damaged duringinstallation of the back cap seal body. The two-part back cap alsoincludes a hollow retainer nut that securely retains the back cap sealbody in position. An outer end of the back cap seal body is contoured toprovide a gap between an outer edge of the back cap seal body and aninner end of the back cap retainer nut. That gap permits the back capseal body to expand under fluid pressure along with the plug valve body,ensuring that the O-ring seal is not stressed when the plug valve issubjected to high fluid pressures. A long service life of the two-partback cap is thereby ensured.

FIG. 3 is a cross-sectional view of a two-part back cap 50 in accordancewith the invention. The two-part back 50 includes a back cap seal body52 and a back cap retainer nut 54. The back cap retainer nut 54 is ahollow nut that includes an inner end 58 having an inner surface 60. Theinner surface 60 is inclined at an angle with respect to an axis of theback cap retainer nut 54, as will be explained in detail below withreference to FIG. 5. The back cap retainer nut also includes a pinthread 59 for engaging box threads of a plug valve body, as will beexplained below with reference to FIGS. 5-7.

The back cap seal body 52 includes an outer end 64 with an inclinedsurface 66. The inclined surface 66 mates with the inner surface 60 ofthe back cap retainer nut only part of the way along the inclinedsurface 66, as will also be explained below in detail with reference toFIG. 5. The back cap seal body 52 further includes an inner end having aplug support surface 62. An outer edge of the inner end further includesa seal rim 68. An outer periphery of the seal rim 68 includes an O-ringgroove 70, which accepts an O-ring 72 for providing a fluid seal betweenthe back cap seal body 52 and a plug valve body of a plug valve inaccordance with the invention. The back cap seal body 52 furtherincludes a seal bore 74, a guide bore 76 and a stud bore 78.

FIG. 4 is a cross-sectional view of a two-part back cap 80 in accordancewith another embodiment of the invention. The two-part back cap 80 isidentical to the two-part back cap 50 described above with reference toFIG. 3, except that it includes a seal rim 82 having a first O-ringgroove 84 and a second O-ring groove 86. The first O-ring groove 84accepts an O-ring 88 and the second O-ring groove 86 accepts an O-ringand 90. As noted above, in all other respects the two-part back cap isidentical to the two-part back cap 50 and the other components will notbe redundantly described.

FIG. 5 is a detailed cross-sectional view of one side of the two-partback cap shown in FIG. 3. As explained above the with reference to FIG.3, the back cap retainer nut 54 has an inner end 58 with an innersurface 60 that is inclined toward the seal rim 68 at an angle ofinclination “a” with respect to a line perpendicular to a central axisof the back cap retainer nut 54. The angle “a” can be in a range from20°-40°. In one embodiment of the invention the angle “a” is 30°. Theback cap seal body 52 has an outer end 64 with an outer surface 66 thatincludes a first surface 66 a adjacent the stud bore 78 (see FIG. 3).The first surface begins at a point 89 and ends at a point 90. The firstsurface is inclined toward the seal rim 68 at an angle of inclinationequal to the angle “a”. The outer end 64 further includes a secondsurface 66 b which is continuous with the first surface 66 a. The secondsurface 66 b extends from the point 90 to an outer periphery of the backcap seal body 52. The second surface 66 b is inclined toward the sealrim 68 at an angle of inclination “b”. The angle “b” may be about0.5°-1.5° greater than the angle “a”. In one embodiment, the angle “b”is about 0.5° greater than the angle “a”. In that embodiment, a gap 92between an outer edge of the back cap seal body 52 and an outer edge ofthe back cap retainer nut 54 is about 0.014″.

As will be explained below with reference to FIG. 7, a differencebetween the angles “a” and “b” is dependent on a rigidity of the plugvalve body 12 into which the two-part back cap 50, 80 is installed. Ingeneral, the more flexible the plug valve body 12, the greater should bethe difference between the angles “a” and “b”. It should be noted thatin one embodiment, the point 90 is aligned with an inner edge of aradius 67 formed at a bottom of an undercut 69 between the plug supportsurface 62 and the seal rim 68.

FIG. 6 is a detailed cross-sectional view of the two-part back cap 50installed in a plug valve 11 in accordance with the invention. Asexplained above, the pin thread 59 on the back cap retainer nut 54engages a box thread in a valve body of the plug valve 11. Only a smallpart of one side of the plug valve 11 is shown. The plug valve 11includes a back cap seal surface 95. The O-ring 72 received in theO-ring groove 70 in the seal rim 68 of the back cap seal body 52 sealsagainst the seal surface 95. When the back cap seal body 52 is installedin the plug valve 11, the back cap seal body is inserted into the plugvalve 11 without rotation. Consequently, the O-ring 72 is not damaged.Thereafter, the back cap retainer nut 54 is installed and tightenedagainst the back cap seal body 52. When the two-part back cap 50 isinstalled in the plug valve 11, a gap 94 exists between the seal rim 68and the seal surface 95. Likewise, the gap 92 exists between outer edgesof the inner surface 60 of the back cap retainer nut 54 and the outersurface 64 b of the back cap seal body 52.

FIG. 7 is a detailed cross-sectional view of the two-part back cap 50shown in FIG. 6 when the plug valve 11 is under high fluid pressure. Asa can be seen, when the plug valve 11 is under high fluid pressure, theback cap seal body 52 “grows” with the plug valve body of the plug valve11 because the seal rim 68 flexes outwardly and downwardly with theincreased fluid pressure. Consequently, the gap 92 and the gap 94 areeliminated and there is metal-to-metal contact between an outerperiphery of the seal rim 68 and the seal surface 95. Likewise, there ismetal-to-metal contact between the surfaces 60 and 66 b. This ensuresthat the O-ring 72 is not stressed at extreme fluid pressures. As aresult, the two-part back cap 50, 80 in accordance with the inventionhas a long service life and rarely requires maintenance, even whenrepeatedly exposed to fluid pressures of up to 20,000 psi. As notedabove, the difference between the angles “a” and “b” is dependent on aflexibility of a plug valve body of the plug valve 11. In general, themore flexible the plug valve body, the greater a difference between thetwo angles should be.

Once installed in the plug valve 11, removal of the back cap retainernut 54 will not remove the back cap seal body 52. Consequently, aplurality of tapped bores 79 which permit the connection of a puller areprovided. Any one of a number of pullers well known in the art may beused to extract the back cap seal body 52 from the plug valve 11.

The invention therefore provides a plug valve 11 that is more reliableduring high pressure fluid service. The plug valve 11 in accordance withthe invention therefore requires less frequent maintenance.

As will be understood by those skilled in the art, the embodiments ofthe invention described above are intended to be exemplary only. Thescope of the invention is therefore intended to be limited solely by thescope of the appended claims.

1. A two-part back cap for a plug valve body, comprising: a back capseal body having an inner end and an outer end, the inner end comprisinga plug support surface for supporting a plug in the plug valve body, andthe outer end comprising first and second surfaces respectively inclinedat respective first and second angles towards a seal rim along an outeredge of the inner end; and a back cap retainer nut that retains the backcap seal body in the plug valve body and has an inner surface that mateswith only the first surface of the back cap seal body to permit the backcap seal body to flex under fluid pressure until the second surfacecontacts the inner surface of the back cap retainer nut.
 2. The two-partback cap as claimed in claim 1 wherein the first angle is 20°-40°. 3.The two-part back cap as claimed in claim 1 wherein the second angle is0.5°-1.5° greater than the first angle.
 4. The two-part back cap asclaimed in claim 2 wherein the first angle is about 30°.
 5. The two-partback cap as claimed in claim 4 wherein the second angle is about 30.5°.6. The two-part back cap as claimed in claim 1 further comprising aradius undercut having an inner edge adjacent the plug support surfaceof the back cap seal body.
 7. The two-part back cap as claimed in claim6 wherein a transition from the first surface to the second surface isaxially aligned with the inner edge of the undercut.
 8. The two-partback cap as claimed in claim 1 wherein the back cap seal body furthercomprises a plurality of tapped bores that permit the connection of apuller.
 9. The two-part back cap as claimed in claim 1 wherein the sealrim comprises an O-ring received in an O-ring groove.
 10. A plug valve,comprising: a back cap seal body having an inner end with a plug supportsurface for supporting a plug received in a plug valve body of the plugvalve and an outer end comprising first and second surfaces inclined atrespective first and second angles; and a back cap retainer nut withthreads that engage the plug valve body to retain the plug and the backcap seal body within the plug valve body, and an inner surface thatsupports only the first surface until the plug valve is subjected toenough fluid pressure to flex the back cap seal body and force thesecond surface against the inner end of the back cap retainer nut. 11.The plug valve as claimed in claim 10 wherein the back cap seal bodyfurther comprises a seal rim along an outer edge of the inner end andthe seal rim supports an O-ring against a seal surface in the plug valvebody.
 12. The plug valve as claimed in claim 10 wherein the back capseal body further comprises a radius undercut having an inner edgeadjacent the plug support surface.
 13. The plug valve as claimed inclaim 12 wherein the second surface is axially aligned with the inneredge of the radius.
 14. A two-part back cap for a plug valve body,comprising: a back cap seal body having an inner end with a valve plugsupport surface, a seal rim that supports at least one O-ring against anend cap seal surface of the plug valve body, and an outer end having afirst surface adjacent a stud bore through the back cap seal body and asecond surface continuous with the first surface, the first surfacebeing inclined toward the seal rim at a first angle and the secondsurface being inclined toward the seal rim at a second angle that isgreater than the first angle; and a back cap retainer nut having athread for engaging the plug valve body to retain the back cap seal bodyand the plug within the plug valve body, the back cap retainer nutfurther having an inner end that supports only the first surface untilthe back cap seal body flexes under fluid pressure and the secondsurface is forced into metal-to-metal contact with the inner end of theback cap retainer nut.
 15. The two-part back cap as claimed in claim 14further comprising a radius undercut having an inner edge adjacent theplug support surface.
 16. The two-part back cap as claimed in claim 15wherein the second surface is axially aligned with the inner edge of theradius.
 17. The two-part back cap as claimed in claim 14 wherein theback cap seal body further comprises a guide bore and a stud bore. 18.The two-part back cap as claimed in claim 17 wherein the back cap sealbody further comprises a stud bore.
 19. The two-part back cap as claimedin claim 14 wherein the first angle is about 30°.
 20. The two-part backcap as claimed in claim 14 wherein the second angle is about 30.5°.